In thin-film deposition systems used for semiconductor manufacturing, a wafer 2, as shown in FIG. 1, is generally held during processing on a pedestal or holder 1 that is situated in a vacuum chamber 5. Material emitted from a deposition source (not illustrated) deposits a thin film on the exposed side of the wafer (which is the intended purpose of the system). However, during the deposition process, some of the emitted material unavoidably is deposited on the surface of components located within the deposition chamber. For example, deposition material 8 is deposited on rings 3 and 4, or shield 6, as shown in FIG. 1. These components 3, 4 and 6 are often designed to be replaced when the accumulated deposited material 8 becomes so thick that it begins to flake off the components and contaminate the deposition process.
In general, due to the properties of the material that is deposited and the deposition conditions, the layers of material deposited on the wafer and on the surface of the components located within the deposition chamber will be in a state of intrinsic mechanical stress. The level of intrinsic mechanical stress depends upon many factors including the particular materials deposited and the relevant deposition process conditions.
In most practical deposition systems, a large number of wafers are processed in sequence. For example, a first wafer is placed in the chamber by a means that maintains the chamber under vacuum conditions. Then, a thin film of material is deposited on the wafer. At the same time, a thin layer of the deposition material is unavoidably deposited on the surface of the components located within the chamber. After deposition on the first wafer is complete, the wafer is removed from the deposition chamber by a mechanism that maintains the chamber under vacuum conditions. Then, a second wafer is placed in the chamber (again by a mechanism that maintains the chamber under vacuum conditions). A thin film of material is deposited on the second wafer, and similarly, another layer of the deposition material is deposited on the surface of the components located within the chamber. This sequence is repeated for many wafers.
During the deposition time for each wafer, an additional thin layer of material 8 is deposited on the surface of the components located within the chamber (for example components 3, 4, and 6) thereby allowing the total accumulated thickness of deposited material on these components to be much thicker than any individual layer deposited on any individual wafer. Unfortunately, the intrinsic stress of the deposited material acts in a way that tends to cause the accumulation of deposited material to pull away from the underlying surface of the components. This tendency to pull away from the surface of the components increases as the total thickness of the accumulated deposited material increases. Whereas, the tendency of the material to pull away from the surface of the components is opposed by the adhesion, if any, between the deposited material and the surface of the components.
It is generally undesirable for the accumulated deposition material 8 to pull away or debond from the surface of the component. If this happens, small particles are created, generally by microscopic or macroscopic fracturing of the accumulated layers of deposited material. If any of these small particles are transferred to the wafers being processed, permanent defects can result in the integrated circuits created on the wafers. In addition, if the particles are large enough, they may interfere with other necessary operations of the chamber components. For example, if the pedestal 1 includes an electrostatic clamp, the clamping action of the electrostatic clamp may be inhibited by the presence of particles between the wafer and the clamp. Once such particle generation has begun from some components located in the chamber, it may be irreversible. As a result, the chamber may have to be opened and cleaned, and various components replaced, in order to restore acceptable processing performance.
Opening of the chamber exposes the interior surfaces of the chamber to the ambient atmosphere. One effect of opening the chamber is that valuable production time is lost since restoring an acceptable vacuum level in the chamber after it has been opened can often require from 6 to 24 hours. Pre-mature opening of the chamber may also force pre-mature replacement of other components which may be quite costly (for example, a partially used sputter target).
In order to process as many wafers as possible between chamber openings, various surface treatments have been applied, in the past, to components such as 3, 4, and 6. It is well known to those skilled in the art that treatments providing a rough surface finish on the components inside the chamber that accumulate deposited material can improve the adhesion of the deposited material to the surface of the components. Such adhesion tends to counteract the tendency of the intrinsic stress of the deposited material which causes the deposited material to pull away from the surface of the component in fractured pieces.
Surface treatment methods such as abrasive grit blasting, bead blasting, arc-spraying, and plasma spraying have all been used in the past for the purpose of providing enhanced surface roughness to improve the adhesion of the deposited film to the treated surfaces of the components inside the chamber. Surface treatment methods such as abrasive blasting or bead-blasting directly roughen the surface material of the component. Methods such as arc-spraying or plasma-spraying add a layer of coating material to the component. In these cases the exposed surface of the coating forms the roughened surface of the finished component.
In many applications, the basic shapes for components such as 3, 4, 6 may be constructed of metal materials. In such cases, the conventional surface roughening treatments usually provide improved performance up to the point at which the deposited material becomes thick enough to finally fracture in spite of the improved adhesion to the surface of the component inside the chamber. However, in some recent applications, components such as 3, 4, and 6 can not be wholly constructed of metal materials due to the unsuitability of the metal's material properties in regard to some other requirements of the processing chamber. For example, if a radio frequency (RF) bias is applied to the pedestal 1 by RF system 7, components near the wafer such as 3 and 4 may be required to have relatively low electrical conductivity on at least part of their surface in order for the deposition process to function correctly.
A common non-conducting material used for such sensitive applications discussed above is aluminum oxide ceramic (alumina). This ceramic material has properties desirable for use in a deposition chamber such as high electrical resistivity, mechanical stiffness and strength, good vacuum properties, ability to be fabricated with accurate dimensional tolerances, etc. However, surface treatments intended to increase surface roughness are more difficult with this hard ceramic material. Thus, abrasive blasting or bead-blasting cannot produce sufficient surface roughness on an alumina component to enhance the adhesion of typical deposited materials to a useful point. Arc-sprayed application of an aluminum coating to the surface of an alumina component that will accumulate deposited material has been successful in some applications. For example, in FIG. 1, the bulk of a component 4 may be conventionally fabricated from alumina ceramic, then coated with a coating 8, which may be aluminum. In these instances, the deposited film adheres well to the arc-sprayed aluminum coating which has a very rough surface.
In some applications, however, a new failure mode appears. In this failure mode, the intrinsic stress in the accumulated deposited material overcomes the adhesion between the arc-sprayed aluminum coating and the surface of the alumina component inside the chamber. Thus, the coating 8 may physically be delaminated from the bulk of the component 4 even though the adhesion of the deposited material to the arc-sprayed aluminum is still sufficient at this point. Unacceptable particle generation will still occur in this case as the combination of the deposited material and the aluminum coating deforms and fractures. Thus, the primary failure mode becomes separation of the aluminum coating from the alumina substrate of the component.
Therefore, what is needed is a method of improving the adhesion between the aluminum coating and the bulk of the component that will substantially extend the number of wafers that can be processed between successive openings of the deposition chamber. Such a method would provide a significant economic advantage over prior art methods and apparatus.